What about an exponentially-scaling "luxury tax" that increases as airlines grow? So there is an underlying "maintenance" cost of running an airline that gets bigger as the airline gets bigger that keeps you making a more linear profit curve rather than making money exponentially as the airline gets bigger.
Basically, at year 5, you can afford 10 A320s per year. In the current landscape, at year 10, you could afford 25,000 A320s per year. With a scaling airline-overhead cost, maybe you could only actually afford 20 A320s in year 10.